1. Field of the Invention
The present invention relates to an improved combination process. The processes are hydrodealkylation of toluene, or toluene and heavier aromatics, and extraction of benzene. Extracted benzene is used to satisfy part of the reflux requirement of the hydrodealkylation unit product fractionator. The inventive concept may also be used when a benzene extraction unit is added to a refinery with an existing benzene fractionator, e.g., an existing aromatics extraction unit with a feed of benzene, toluene and xylenes.
The invention is specifically described with reference to thermal hydrodealkylation of a toluene fraction, and extraction of benzene from non-aromatics in an extraction unit using Sulfolane solvent.
2. Description of the Prior Art
Hydrodealkylation (HDA) of toluene and heavier alkylaromatics to benzene is a well known process. In a typical dealkylation unit, toluene is contacted with hydrogen, perhaps in the presence of a catalyst, at high temperature to remove the methyl group from the toluene feed, thereby producing benzene. An excellent overall view of this process is presented in Kirk-Othmer, Encyclcopedia of Chemical Technology, Second Edition, Inter-Science Publishers, N.Y., 1966, Volume 11, page 453, entitled "Hydrodealkylation." The teachings of this article are incorporated by reference.
Typically, less than 100% conversion of the feed to desired product occurs in the reaction zone, so a fractionator is usually provided to separate benzene from unconverted toluene to permit recycle thereof to the reaction zone. It is also conventional in many units, to provide clay treating of the feed to the dealkylation unit fractionator. The purpose of clay treating is to remove trace amounts, typically 10 to 5000 wt ppm, of olefins present in the dealkylation effluent. Clay treating polymerizes these olefins and permits their removal from the benzene product by fractionation. It is not possible to simply clay treat the benzene product after fractionation because, although the olefins would be polymerized, the polymer would still be present in the benzene and would seriously degrade the color of the benzene product, necessitating further fractionation to remove polymers.
Examples of typical dealkylation units are given in U.S. Pat. Nos. 3,204,007 and 3,204,006, both in Class 260-672. The teachings of these patents are incorporated by reference. U.S. Pat. No. 3,204,007 discloses a dealkylation unit wherein reactor effluent passes through heat exchangers and coolers into a vapor-liquid separator. The liquid fraction is clay treated and then fractionated to produce benzene product and toluene for recycle.
U.S. Pat. No. 3,204,006 discloses a dealkylation unit with feed pretreatment facilities which permit desulfurization of the feed in a hydrorefining zone, and also removal of nonaromatics from the feed. In this patent, it is contemplated that the product fractionator will also fractionate a feedstock containing some aromatics. To the extent that this patent provides for fractionation of mixtures of reactor effluent and outside benzenes in a single fractionation column, the treatment of these streams in the distillation column is as a gross mixture thereof.
Aromatics extraction is also a very old process. Typically these units separate a catalytic reformate or a coke oven light oil, via liquid-liquid extraction or extractive distillation or both, into aromatics and non-aromatics. The development of one commercial process has been reported at the 7th World Petroleum Congress, Volume IV, pages 65-73, 1967, which article is incorporated by reference.
In the past it was customary to treat a feed in an aromatics extraction unit which contained not only benzene but also toluene and xylenes and perhaps heavier aromatics as well. With the trend toward higher severity reforming, the amount of non-aromatics which survive the reforming zone has diminished. Recently, the amount of non-aromatics boiling in the toluene and xylene fractions has been low enough to permit the toluene fraction to be fed directly to a hydrodealkylation unit, and the C.sub.8 aromatics to be fed directly to conventional xylene extraction and separation processes, all without aromatics extraction. Typically, non-aromatic content of the C.sub.7 fraction will be less than 10%, preferably less than 5%. Accordingly, the need for aromatics extraction capacity treating feeds heavier than benzene has seriously diminished. Many refiners require aromatics extraction of only the benzene fraction from a reformate. Unfortunately, the benzene product obtained from such a specialized extraction unit, although fairly pure, still requires expensive fractionation downstream of the extraction unit.
Fractionation is required because the benzene product from the aromatics extraction unit is inevitably contaminated with trace amounts of olefins. These contaminants, if not removed, would cause the benzene to fail typical acid wash color tests, e.g., ASTM D-848. The traditional way of removing these contaminants is to pass the benzene product over a bed of clay, which polymerizes the olefins and permits their separation from benzene by fractionation. This is a relatively simple fractionation, because there is a great difference in boiling point between benzene and polymer. However, a substantial investment is required in even a simple fractionation column, and in addition to the reflux requirement, the entire benzene product must be vaporized and this increases the utility costs, as is well known by those skilled in the aromatics extraction art.